MECHANISM OF PAIN PERCEPTION
How do we feel pain?
Microscopic structures called pain receptors are present throughout our body. When a painful event happens, the damaged area releases chemicals, which excite the C-type nerve fiber nociceptors. They transmit the pain signal to the spinal cord area called the dorsal horn. There, the pain signal intensity is filtered, modified and sent to the brain through a bundle of nerve fibers called the spinothalamic tract. See explanation of spinothalamic tract.
The brain area called the somatosensory cortex controls the sensory aspect of the pain and certain areas of hippocampus and amygdala control the emotional component of pain.
In addition, the brain also releases the body’s own painkillers called endorphins and enkephalins which act in the injured area to decrease the intensity of the pain.
Mechanism of Chronic Pain
In chronic pain, pain signals continue to be generated in the injured area long after it has healed. It results in permanent pain processing changes in the brain and spinal cord giving rise to chronic pain.
To be more specific, the peripheral nerve fibers convey pain signal to spinal cord using a chemical called Glutamate. It binds to AMPA receptors present in the dorsal horn of spinal cord like a lock and key thereby transmitting the pain signal to spinal cord and brain. In chronic pain, repetitive signals from the periphery produces excess glutamate at which point it binds with another set of exposed new receptors called NMDA receptors. NMDA receptor activation results in hyper-sensitization of brain and release of a pain-amplifying chemical called Substance P. At this stage, the acute pain changes into chronic pain.
Even if you are unable to understand the above mechanism, do not bother. Just understand that the mechanism and therapeutic goals of acute and chronic pain are different.
FACTORS AFFECTING PAIN – what are they?
The factors affecting pain are
- Emotional status
Two specifically vulnerable groups to pain are children and the elderly. Children, contrary to popular belief, are more sensitive to pain than adults.
It is because
- They show more inflammatory response to pain
- The pain signals reach their brain in full intensity without getting modified by the dorsal horn of spinal cord
- Their descending pain inhibiting pathways from brain are less developed than in adults
Children, moreover, are unable to communicate pain under the age of two, which frequently result in them being under treated, for their pain.
Similarly, elderly people are more sensitive to pain because of their diminishing bone mass and generalized age related wear and tear of their body. They are also under treated often for their pain because some of them think that pains and old age go together. Their doctors sometimes hold back their pain medicines for fear of increased incidence of side effects.
Women are more prone to experience pain at levels, which men usually tolerate because of the difference in sex hormones and increased prevalence of depression and anxiety in them.
Sometimes, muscles learn to transmit pain signals even after the underlying injury to the bone or joint heals. It is because; they undergo tightening to protect the injured area as soon as it occurs. When inadequate physiotherapy and conditioning exercises are done, they continue to remain so. This results in the patient continuously experiencing pain.
People in general suffering from depression and anxiety are prone to experience more pain.
Smoking individuals experience more pain and obtain less relief with pain medications. It is because nicotine reduces body stores of wound healing vitamin C, adversely affects the pain processing mechanisms of the brain and interacts with opioid pain medications.
People who have experienced childhood injuries and trauma show more pain sensitivity when they experience pain in adulthood.
People experience arthritic pain more during winter season.
Sleep deprivation increases pain perception.
Causes of pain
There are a number of factors which can cause pain. But whatever may the cause the pain signals ultimately travel along the nerve fibers found along the peripheral portion of the spinal cord called the “spinothalamic tract”. This tract finally ends in the sensory cortex of brain which makes the person perceive pain.
Similarly all the different pains have a common basic mechanism of signal production at the cellular levels. At the tissue levels the pain is classified in to neuropathic pain that arises due to damage to the nervous tissues and nociceptive pain that arises from damage to tissues other than nervous tissues. So all the different types of pains that are mentioned above can be placed under these two groups.
The neuropathic pain as mentioned earlier is due to the damage to the nervous tissue and can be either Central neuropathic pain when brain and spinal cord are injured or peripheral neuropathic pain when the peripheral nervous system is injured.
Causes of central neuropathic pain:
Some of the causes of central neuropathic pain are
- Nerve compression from occlusion of the opening in the spine
- Multiple sclerosis
- Parkinson’s disease
- Post stroke pain
- Post-traumatic spinal cord injury pain
Causes of peripheral neuropathic pain:
Some of the causes of peripheral neuropathic pain are
- Cervical, if neck is involved
- Lumbosacral, if low back is involved
- Peripheral diabetic neuropathy
- Complex Regional Pain Syndrome
- Post-herpetic neuralgia
- Trigeminal neuralgia
- Phantom limb pain
- Alcoholic polyneuropathy
- Carpal tunnel syndrome
The specific changes associated with centrally-generated pain syndromes are not known. But some of the mechanisms of pain in the nerves in the peripheral neuropathic type are understood. Injuries to nerve fibers result in regeneration of sprouting nervous tissues called neuroma. These neuroma can generate spontaneous activity and they have increased sensitivity to physical distension. This nerve pain is usually felt as tingling pains when the area associated with it is tapped or manipulated.
Nociceptive pain as mentioned earlier is the pain resulting from damage to tissues other than nervous tissue and it comprises two types of pain – somatic and visceral.
Causes of somatic nociceptive pain:
- Ligament strain
- Degeneration of cartilages of the joint
- Fracture of bones
- Chronic myofascial pain
Causes of visceral nociceptive pain:
- Renal colic
- Biliary colic
- Ureteric colic
- Pleuritic pain
Nociceptive pain consists of four processes-
When a tissue is damaged, it stimulates the pain receptors or the nociceptors by a process called transduction. The signals are then transmitted along the spinal cord to the brain. Only after the signals reach the brain, it is perceived as pain. The transmission of these pain signals is modified along its pathway by certain substances like endorphins.
Nociceptive pain can also result from inflammation. The inflammation pain is due to the release of chemical substances such as Substance P, serotonin, histamine, acetylcholine, and bradykinin. These substances activate and sensitize other nociceptors.